The need for an efficient thermal energy storage means in a solar energy or any other heat energy collection system in which thermal energy is to be stored and subsequently recovered for use has long been recognized. Water and stones have been used successfully but require large storage areas because of their low heat capacities. Salt hydrates because of their high heat capacity at the transition point between hydrated and unhydrated crystal states provide an excellent heat storage material. These materials efficiently absorb and release thermal energy when first used. However, after continued cycling, exothermic characteristics of a salt hydrate change due to stratification, and recrystallization is retarded. A slow rate of recrystallization through a large hydrate mass limits the rate of heat absorption between the layer closest to the heat source and inner layers. Laboratory tests have indicated that continued mixing combined with an addition of nucleating agents to promote recrystallization, solves some of these problems. Increasing the added to a heat transfer means in a lower portion of the bath liquid creates a convection current which is cooled as it flows past the heat storage material containers. At an upper level of the bath liquid, a baffle means is provided to direct the convection current into a downwardly moving portion so that movement of the convection current causes the rotatably mounted containers to continually rotate. The containers have inwardly extending protrusions which continually agitate the heat storage material contained therein as the containers rotate, thereby preventing stratification and increasing efficiency of the heat transfer process as previously explained.
In specific embodiments of the invention, the container is positioned in an upwardly moving portion of the convection current and is in the form of a longitudinally-extending tube whose outer surface is configured so that a rotating moment is applied to the container as the convection current flows. In one embodiment of the invention, the outer surface is formed in the shape of longitudinally-extending protrusions or ridges which are angled with respect to the surface of the container so that equal flow on each side of the container imparts unequal moments to the container. In another embodiment of the invention, the outer surface of the container forms longitudinally-extending grooves or notches which are also configured so that equal current flow on each side of the container imparts unequal moments to the container. Use of a thermal energy storage apparatus provided by the invention is disclosed in conjunction with both a solar collector and a thermal radiator.
In a heating or solar energy collection application, a salt hydrate having a transition point between hydrated and unhydrated crystal states of about 95.degree. F. is used, one example of such a salt hydrate being sodium sulfate (Na.sub.2 SO.sub.4.10H.sub.2 O). The sodium sulfate and its associated container are chosen to have a specific gravity of approximately 1.2, and a heat transfer bath of anhydrous calcium chloride (CaCl 30%-70% water) is chosen to have a specific gravity of about 1.3. In a cooling application such as an air conditioning system, a thermal energy radiator is used for removing heat from the thermal energy storage apparatus at night, the heat storage material having a transition point between hydrated and unhydrated crystal states of approximately 57.degree. F. An example of such a heat storage material would be sodium hydroxide (NaOH.31/2H.sub.2 O). In this application, heat is removed from the top of the thermal energy storage apparatus by a thermal energy radiator, and it is added to the bottom of the thermal pile by a flowing fluid to be cooled .